compile_warning!

Related to https://github.com/rust-lang/rfcs/issues/2902 and https://internals.rust-lang.org/t/pre-rfc-add-compile-warning-macro/9370, I want a compile_warning! macro that is similar to compile_error! except that it is not a hard error.

Here is a motivating use case. Say I have two barely compatible features, I would expect to be able to emit a warning based on my knowledge as a developer and experience with interface symmetry. However, it is not possible to do this:

#[cfg(all(feature = "feature_a", feature = "feature_b"))]
compile_warning!("Using 'feature_a' and 'feature_b' together is not recommended.");

There are many reasons why it may not be possible to have truly mutually exclusive features in Rust. I hit this a lot when trying to divvy up the limited static memory on microcontrollers.

I think the #[allow] and #[deny] points in the threads very relevant. It would be good feedback to someone building the crate to see the warning once, then be able to silence the warning. For example:
In a library crate:

#[cfg(all(feature = "feature_a", feature = "feature_b"))]
compile_warning!(foobar, "Using 'feature_a' and 'feature_b' together is not recommended.");

In the consuming crate:

// We checked we want this ill-advised configuration
#[allow(foobar)]

Start by reading RFC 2902 and the linked pre-RFC discussion, then compare the requested compile_warning! behavior with compile_error!, including the proposed #[allow] and #[deny] handling. Done would require a settled design for warning emission, feature-gated use cases, and suppression in consuming crates; this issue does not name implementation files or tests.